Method for fabrication of vertically coupled integrated optical structures

ABSTRACT

A manufacturing process is provided for fabrication of vertically coupled integrated photonic devices by projection lithographic technique. A multi-layered structure is formed which includes a pair of core waveguiding layers separated by a coupling interlayer and sandwiched between cladding layers. Prior to forming optical features in the core layers, alignment marks are etched completely through the whole multi-layered structure with the alignment marks being visible on both sides of the multi-layered structure to a conventional projection stepper. After the alignment marks are formed, a “bottom level” optical features are made through the bottom cladding layer, bottom core layer, and portion of intervening coupling layer. The formed sample is then bonded by a polymer to a carrier and a “top level” optical features are defined through the top cladding, top core layer, and portion of the intervening coupling layer.

[0001] This invention was made with Government support under ContractMDA90497C0476 awarded by the National Security Agency. The Governmenthas certain rights in this invention.

FIELD OF THE INVENTION

[0002] The present invention relates to fabrication of integratedphotonic devices; and particularly, to a technique for fabrication ofintegrated optical devices in which highly confined optical components,such as optical waveguides, micro-ring resonators, couplers,interferometers, etc., are coupled vertically, as opposed to lateralcoupling.

[0003] Even more particularly, the present invention relates to atechnique for fabrication of vertically coupled integrated opticaldevices by applying the advantages of projection photolithography,substrate removal and low temperature polymer binding to attain opticalintegrated structures of high quality.

[0004] The present invention is further directed to a method forfabrication of various integral optical waveguide systems employing atechnique for deep etching of alignment marks thus attaining increasedalignment quality of the device's design which makes it possible tofabricate vertically coupled highly confined optical waveguide devicesby means of conventional projection lithography in a process compatiblewith low cost mass production.

[0005] The present invention further is related to fabrication ofintegrated optical waveguide devices which comprise at least twowaveguiding core regions separated by an inner layer region which issandwiched between cladding regions. All regions of the device extend invertical mutual disposition to form a vertically coupled ensemble. Theentire ensemble is then encapsulated within a polymer. Independentoptical components of the device are formed in the waveguide coreregions by etching of waveguiding core regions in a predeterminedsequence in accordance with a desired pattern wherein the relativealignment of the formed optical components is achieved by special deeplyetched alignment marks which are fabricated prior to the etching of thewaveguiding core regions.

BACKGROUND OF THE INVENTION

[0006] Integrated photonics has proved to be a useful technology fortransmitting, receiving, routing and processing of information inoptical form as is widely documented in an extensive literature.Typically, integrated photonic devices require the controlled couplingof light between waveguides formed in an integrated chip. It is known inthe prior art that a useful method for performing this coupling is toposition two waveguides in parallel fashion with a controlled separationallowing light to couple the waveguides 1 and 2 across the gap 3 formedtherebetween as shown in FIG. 1. The properties of this type of couplingare determined by the material, shape and dimensions of the waveguides,cladding parameters, and the dimension of the coupling gap 3 which isparticularly important in determining the coupling strength between thewaveguides 1 and 2.

[0007] Typically, integrated photonics fabrication is based on theplanar processing technology developed for semiconductor integratedcircuits, while layers of material are deposited or grown upon largelyplanar substrate surfaces. Structures are patterned along the surface ofthe plane using a succession of lithographic processes combined withvarious etching, deposition, alloying, implantation and other well-knowntechniques.

[0008] Integrated optical waveguides may be fabricated in the followingways: a) with dimensions perpendicular to the plane (which is referredto herein as the vertical direction) generally determined by thethickness of grown or deposited layers with values being set by thegrowth or deposition process parameters, or (b) with the in-planedimensions (which are referred to as lateral directions) generallydetermined by lithographic patterns as they define the regions whereother additive, subtractive or modifying processes are applied.

[0009] It has been noted that coupling of two waveguides 4 and 5 in amanner that the waveguide separation, or coupling gap 6 is oriented inthe vertical direction (referred to as vertical coupling) shown in FIGS.2A and 2B has several advantages.

[0010] Initially, the waveguide separation in vertically coupledstructures is determined by a layer growth or deposition process ratherthan by a lithographic process. Since it is frequently possible tocontrol the thickness and material characteristics of a grown ordeposited layer to a higher degree of accuracy and precision than thatof a lithographic process, an enhanced control over the couplingstrength between vertically separated waveguides is attained.

[0011] Secondly, vertical coupling geometry has an advantage of greatlyenhanced alignment tolerance between the waveguides to be coupled. Asdescribed previously, the coupling strength between two waveguides is ahighly dependent function of the waveguide separation and in many casesthe relationship has an exponential dependence.

[0012] In conventional integrated couplers in which two waveguides 1 and2 are separated in a lateral direction (as shown in FIG. 1), arelatively small variation in the waveguide separation 3 due tolithographic nonuniformity or misalignment may cause a considerablechange in the coupling strength. In the case of vertical coupling shownin FIGS. 2A and 2B, the maximum coupling strength occurs when twowaveguides 4 and 5 are optimally aligned in the lateral direction. Atsuch an optimum point, the dependence of coupling on misalignment(lateral displacement 7) is stationary to a first order (the firstderivative is zero at a maximum point). Thus, the coupling can be maderelatively insensitive to small errors in the lateral alignment 7 ofvertically coupled waveguides.

[0013] Microring resonators proved to be promising building blocks forvery large-scale integrated optics. In the past few years, singlemicroring resonators laterally coupled to a waveguide have beenfabricated in Si-SiO₂ and GaAs-AlGaAs. Advanced functions thereof havebeen demonstrated such as high-order filtering for DWDM applications,notch filters and wavelength conversion. These devices were based onlateral coupling between the waveguides and the ring fabricated bylithography. In these devices it is critical to make the separationbetween the waveguides and the microring smaller than 0.3 μm.Disadvantageously, lithography may fail to deliver such preciseness ofparameters, thus making reproducible bandwidth and high power droppingefficiency of the integrated optical devices difficult to achieve.

[0014] This problem is substantially alleviated with vertical couplingby controlling the sensitive separation between optical components withmaterial growth or deposition, and incorporating the inherentlysymmetric structure of the integral devices. Waveguides and ringresonators that take advantage of vertical coupling have beendemonstrated which were fabricated in compound glass, silica and insemiconductors. The process included sequential deposition (regrowth) ofseveral waveguiding layers, one on top of another, with patterning ofeach layer before deposition (regrowth) of a subsequent layer. Thistechnique required multiple redeposition (or regrowth) and planarizationwhich has suffered from unwanted complexity.

[0015] A regrowth-free fabrication technique was developed which allowedfabrication of vertically coupled structures in semiconductors usingwafer bonding, substrate removal and infrared backside contactalignment. The regrowth-free technique, however has suffered from anumber of drawbacks. First, the method used contact lithographytechniques outfitted with infrared alignment systems which are able todo accomplish alignments and exposures, however, the method has seriouslimitations in resolution and inter-level alignment accuracy as comparedto projection systems. Due to the limitations in resolution andalignment presented by infrared backside-aligned contactphotolithography, it was difficult to attain highly confined verticallycoupled structures.

[0016] Secondly, due to the fact that alignment marks on the originaltop surface of the structure would not be visible to a conventionalprojection stepper after the wafer bonding and substrate removalprocess, the direct write electron beam lithography or deep UVphotolithography was necessitated to achieve a deep sub-micron alignmenttolerance which was still a requirement between waveguides even in thecase of vertically coupled devices. Thus, this technique was notcompatible with projection lithography which is a known technique wellsuited for widespread use of large scale silicon integrated circuitfabrication.

[0017] It has become apparent that a simpler technique for fabricationof integrated photonic devices compatible with projection lithographyfor manufacturing of highly confined waveguides in semiconductors withthe sub-micron alignment tolerance between waveguides is still necessaryin the field of integrated photonics.

SUMMARY OF THE INVENTION

[0018] It is an object of the present invention to provide a fabricationprocess for manufacturing of vertically coupled optical integratedwaveguide devices having a high degree of vertical confinement andisolation of the optical elements by means of projection lithographymeeting the requirement of sub-micron alignment tolerance between theoptical elements.

[0019] It is another object of the present invention to provide aprocess for fabrication of vertically coupled optical waveguideintegrated devices using deeply etched alignment marks visible to aconventional projection I-line projection stepper where the alignmentscheme of the present invention provides for a high quality layer to alayer alignment with an alignment tolerances limited to below 0.25microns.

[0020] It is still an object of the present invention to provide amanufacturing process for fabrication of vertically coupled opticallyintegrated structures comprising at least two waveguide core layersseparated by interlayers and sandwiched between cladding layers whichare encapsulated as an entire ensemble within a low index polymer. Sucha structure allows each of the waveguiding cores to be individuallyoptimized in dimension and composition for their particular function. Inthis manner, the technological process provides for complete patterningaccess to the waveguiding layers without incurring the difficultiesencountered with a single side patterning approach of the prior art.

[0021] According to the teaching of the present invention, a method forfabrication of vertically coupled integrated photonics devices isprovided which comprises the steps of:

[0022] (a) forming a multi-layered structure having at least a pair ofwaveguiding core layers separated by a coupling layer and sandwichedbetween a pair of cladding layers,

[0023] (b) forming alignment marks extending through the whole width ofthe multi-layered structure,

[0024] (c) forming first waveguiding features at the bottom level of themulti-layered structure by through etching the respective claddinglayer, waveguiding core layer and partially through the coupling layerin accordance with a first pattern, and

[0025] (d) forming second waveguiding features at the top level of themulti-layered structure by through etching another cladding, waveguidingcore layer and partially the coupling layer in accordance with a secondpattern so that at both the bottom and the top levels, the features arealigned to the alignment marks which are easily identifiable at both thetop and bottom surfaces of the multi-layered structure by a conventionalprojection stepper. Projection lithography techniques are well-suited tothe method of the present invention.

[0026] It is important that the coupling layer is not removed completelyby etching the waveguiding features on the top and bottom levels of themulti-layered structure. Preferably, the remaining portion of thecoupling layer (portion not removed) is not thicker than 0.3 μm whichprovides optical isolation between the waveguiding features as well asadding mechanical strength to the structure.

[0027] After the features of the bottom level of the multilayeredstructure have been defined as provided in step (c), the entirestructure is covered with a low refractive index polymer. The substrateon which the multi-layered structure is deposited or grown, is removedin order to spare the top level surface of the multi-layered structurefor further patterning by means of projection lithography. After the toplevel features have been defined, the low refractive index polymer ispositioned on the top level features which completely envelopes theentire device therein.

[0028] Viewing another aspect of the present invention, there isprovided a vertically coupled integrated photonic device which includes:

[0029] a multi-layered structure having at least a pair of waveguidingcore layers patterned independently and in predetermined sequence todefine a top level and bottom level waveguiding features, respectively,

[0030] a coupling layer spacing said top layer and bottom layer levelwaveguiding features,

[0031] a pair of cladding regions sandwiching the waveguiding layerstherebetween, and

[0032] a low refractive index polymer completely encapsulating themulti-layer structure therewithin.

[0033] It is important to the concept of the photonic device of thepresent invention that a portion of the coupling layer always remainsbetween the vertically spaced waveguiding core layers for increasedoptical isolation therebetween as well as to add mechanical strength tothe overall structure. The overall thickness of the coupling layer isgenerally maintained to a dimension of approximately 0.3 microns orless.

BRIEF DESCRIPTION OF THE DRAWINGS

[0034]FIG. 1 is a schematic representation of a laterally coupledwaveguide structure of the prior art;

[0035]FIG. 2A is a perspective view of a vertical waveguide coupler ofthe prior art;

[0036]FIG. 2B is a schematic representation of the vertical waveguidecoupler of FIG. 2A;

[0037]FIG. 3 is a schematic cross-section of a multi-layer structure inwhich a vertically coupled integrated optical device of the presentinvention is formed;

[0038]FIGS. 4a-4 d show schematically the succession of technologicalsteps for forming the alignment marks in the multi-layer structure ofthe present invention;

[0039]FIGS. 5a-5 d show schematically the succession of technologicalsteps for forming “bottom level” waveguide patterns in the multi-layerstructure of the present invention;

[0040]FIGS. 6a-6 b show schematically the substrate removal and polymerbonding of the structure shown in FIG. 5d to a carrier substrate;

[0041]FIGS. 7a-7 c show schematically the sequence of operations of thetop level patterning and the carrier removal;

[0042]FIG. 8 is a top view of a vertically coupled GaAs/AlGaAs singlemicroring resonator add/drop filter of the present invention;

[0043]FIG. 9 is a top view of 1-by-4 crossbar MUX/DEMUX with verticallycoupled GaAs/AlGaAs double microring resonator of the present invention(the dotted lines show the contours of the underlying guidingstructure);

[0044]FIG. 10 is a diagram showing a normalized coupling coefficient asa function of the waveguide's misalignment for a) the coupling layercompletely removed (dotted line) and for b) the coupling layer of 0.3micron thickness remained;

[0045]FIG. 11 is a diagram representative of the spectrum of the singlemicroring resonator add/drop filter of FIG. 8;

[0046]FIGS. 12A and 12B are diagrams representative of normalized powervs. wavelength for fabricated microring resonators of FIG. 8,specifically, FIG. 12A represents a resonance comparison between TM andTE polarization, while FIG. 12B represents throughport and drop port TMspectra;

[0047]FIG. 13 is a diagram showing a normalized dropped power vs.wavelength in the drop port 1, drop port 2, drop port 3 and drop port 4of the 1-by-4 MUX/DEMUX of FIG. 9, fabricated in accordance with thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0048] Referring to FIG. 3, the fabrication process of the presentinvention is initiated by formation of an initial multi-layer structure10 positioned above a semiconductor substrate 11 which is separatedtherefrom by an etch stop layer 12 formed on the top of thesemiconductor substrate 11. The multi-layered structure 10 includes afirst cladding layer 13 formed on the top of the etch stop layer 12, afirst core layer 14 formed on the top of the first cladding layer 13, acoupling layer 15 formed on the top of the first core layer 14, a secondcore layer 16 formed on the top of the coupling layer 15 separated fromthe first core layer 14 by the coupling layer 15, and a second claddinglayer 17 formed on the top of the second core layer 16.

[0049] Thus formed, multi-layer structure 10 includes a pair of corelayers separated by the coupling layer and sandwiched between a pair ofcladding layers. It is important to the understanding of the presentinvention that formation of more than two core layers separated by arespective coupling layer is also contemplated in the scope of thepresent invention and can be easily performed for the photonic integraldevices which would include more than two vertically coupled waveguidestructures.

[0050] Therefore, it is clear that the structure of FIG. 3 which wouldcomprise a pair of core layers 14 and 16 separated by the coupling layer15 is presented for the sake of clarity and ease of understanding ofthis particular example of the fabrication process and the structure ofthe present invention and not for the purposes of limiting the scope ofthe invention.

[0051] The layer thicknesses and compositions are chosen to provideoptical waveguiding in the higher refractive index core layers 14 and 16within a lower index cladding layers 13 and 17. The refractive index andthickness of the coupling layer 15 are chosen to provide an appropriateamount of coupling between a waveguide feature formed in the first corelayer 14 and a waveguide feature formed in the second core layer 16 forthe range of waveguide transverse dimensions to be used in the exampledevice, which is intended to produce single transverse mode waveguidesin the completed device. Optical waveguide mode software is used todetermine the approximate values of the layer and device designparameters which can be finely tuned through experiment.

[0052] In the example device discussed herein, a GaAs substrate was usedwith the approximately 600 microns thickness, on the top of which a 0.1micron In_(0.48)Ga_(0.52)As etch stop layer 12 was epitaxially grown ordeposited by techniques known to those skilled in the art. A firstcladding layer 13 of 0.3 micron Al_(0.5)Ga_(0.5)As was epitaxially grownor deposited on the top of the cladding layer 13. 0.5 micron GaAs firstcore layer 14 was epitaxially grown or deposited on the top of the firstcladding layer 13 and the 0.5 micron Al_(0.5)Ga_(0.5)As coupling layer15 was grown or deposited on the top of the first core layer 14. Then,0.5 micron GaAs second core layer 16 was epitaxially grown or depositedon the top of the coupling layer 15, and finally a second cladding layer17 of 0.3 microns Al_(0.5)Ga_(0.5)As was grown or deposited on the topof the second core layer 16. Multi-layer structure 10 as providedconstituted a symmetrical structure where the axis of symmetry extendedthrough the coupling layer 15. It is however to be understood thatdespite of the fact that in the example structure shown in FIG. 3 havinga pair of identical first and second core layers 14 and 16, as well assubstantially identical first and second cladding layers 13 and 17 (bothcomposition and thickness-wise), these layers are independent each fromthe other and the first and second core layers, as well as first andsecond cladding layers can be made with different and distinctcomposition content as well as different thickness, as needed for anintended integral photonics device. The initial multi-layer structures10 shown in FIG. 3 after being formed, are cleaved along crystal planesinto convenient sized pieces (13.5 mm) for subsequent processing as willbe described in further paragraphs.

[0053] After the multi-layer structure 10 has been formed, alignmentmark patterns are created. It is an important feature of themanufacturing process of the present invention that the alignment markpattern is formed to extend substantially through the entire multi-layerstructure 10, i.e., extending through the first cladding layer 13, firstcore layer 14, the coupling layer 15, the second core layer 16, and thesecond cladding layer 17 as best shown in FIGS. 4B-4D.

[0054] Such a formation of the alignment mark pattern is an importantfeature of the manufacturing process for fabrication of integralphotonic devices with vertically coupled optical components, since ittakes advantage of conventional projection lithography techniques aswell as giving independent access directly to both core layers of thestructure for forming (independently each from the other) waveguidingpatterns in the core layers.

[0055] In order to form the alignment marks (or alignment keys) 18, aphotolithography was performed on the multilayer structure 10 using 2.1micron thick I-line positive photoresist, exposed on a 10×I-line stepper(lens N.A. 0.315). This process is well-known to those skilled in theart and therefore is not discussed herein in detail.

[0056] As shown in FIG. 4A, a positive photoresist 19 was deposited onthe top of the cladding layer 17 and was exposed through a mask whichcontained a pattern corresponding to reference alignment marks suitablefor the alignment of subsequent patterns as well as reference verniersfor evaluation of alignment accuracy. Resist backing and developmentwere done according to the resist manufacturer's recommendations. Thestepper focus and exposure values have been optimized prior to the usein the process of the present invention using a resolution test pattern;the stepper base line alignment was also checked. The sample waspositioned on the stage and aligned with a crystal cleavage planeparallel to the stepper stage travel to facilitate later cleavage of thedevice bars as known to those skilled in the art.

[0057] After the alignment marks pattern has been formed in thephotoresist 19 on the top of the cladding layer 17, as shown in FIG. 4A,the samples were etched to define the alignment marks 18 through thelayers (first and second cladding layers 13 and 17, first and secondcore layers 14 and 16, and the coupling layer 15), as shown in FIG. 4B.The etching depth is sufficient (2.1 microns) to allow the alignmentmark 18 to be seen from the front surface 20 as well as from a backsurface 21 of the multi-layer structure 10 after removal of thesubstrate 11 and the etch stop layer 12, as will be described infollowing paragraphs. Etching was performed using chlorine/borontrichloride inductively coupled plasma (ICP) etching, using thephotoresist as a mask.

[0058] After the alignment marks 18 have been etched through themulti-layer structure 10, a metal film 22 consisting of titanium (10 mmlayer) and gold (90 nm layer) were deposited on the sample usingelectron beam evaporation, as shown in FIG. 4C. The sample was placed inwarm n-methyl pyrolidinone (NMP) to dissolve the photoresist mask 19 andlift off or remove the area of metal film 22 on the top surface 20 ofthe sample. Metal film 22 remained within the etched alignment marks 18,specifically on the bottom thereof, as shown in FIG. 4D, and contributedto the high visibility of the alignment marks 18 in subsequent alignmentsteps. The sample was further cleaned using solvent rinsing in acetone,methanol and isopropanol.

[0059] After the alignment marks 18 have been formed, thephotolithographic process was performed to form a “bottom level” of theoverall device features. Referring to FIG. 5A, a layer of thephotoresist 23, aligned to the sample alignment marks 18 was depositedand exposed through a photo mask to define the “bottom level” waveguidefeatures.

[0060] The pattern formed in the photoresist level 23 was designed toleave the photoresist in the areas 24 where the optical waveguidefeatures were intended to be formed in the core layer 16, as shown inFIG. 5B. The bottom level waveguide pattern were then etched using ICP(inductively coupled plasma) etching to a depth of 0.95-1.0 micron usingthe photoresist 23 as a mask. This etching step removed the secondcladding layer 17 in predetermined areas, second core layer 16, and aportion of the coupling layer 15 to form highly confined waveguidefeatures 25, as shown in FIG. 5C. After the etching, the photoresist 23was removed to form the sample 26 with the “bottom level” waveguidefeatures 25 extending from the coupling layer 15,d as shown in FIG. 5D.

[0061] The etching was performed in a three step process:

[0062] 1. The sample 26 was exposed to an oxygen sample cleaning processin a reactive ion etching chamber for two minutes;

[0063] 2. The sample 26 was soaked in warm n-methyl pyrolidinone (NMP)at 70 centigrade for at least 10 minutes, and then rinsed with organicsolvents, such as acetone, methanol, and isopropanol; and

[0064] 3. The sample 26 was run for 2-3 minutes through the oxygenplasma strip process at high pressure and high power to remove residualphotoresist 23 on the features 25.

[0065] After the “bottom level” features have been formed, a GaAs chip27 was prepared to be used as a carrier, which was cleaved to besomewhat larger in area than the sample shown in FIGS. 3-5D. The carrier27 was cleaned using an organic solvent rinse, and an adhesion promoter(Dow Chemical Company AP3000 Cyclotene Adhesion Promoter) was applied tothe top surface of the GaAs carrier 27, which was spun at 2000 rpm(rotations per minute) for the duration of 30 seconds. The adhesionpromoter was also applied to the sample in process to cover the “bottomlevel” waveguide features 25. The adhesion promoter applied to thesample was also spun to uniformly spread it over the sample 26.

[0066] Further, a predetermined quantity, specifically a drop ofbenzocyclobutene (BCB) Dow Chemical Cyclotene 3022-57 was deposited onthe GaAs carrier 27 and the device sample 26 was inverted with features25 being downwardly extending and placed onto the carrier 27, as shownin FIG. 6A, with the crystal axis of the carrier 27 and the sample 26roughly pre-aligned. Both the sample 26 and the carrier 27 were gentlypressed to provide a uniform BCB layer 28 between the sample 26 and thecarrier 27. Excess BCB was removed from the sides of the carrier 27, andcareful handling was used to insure that the entire top surface of thecarrier 27 was covered with the BCB 28 to act as a protection layer insubsequent wet etching.

[0067] Care was taken to maintain the unbonded surface 29 of the carrier27 free of BCB. The ensemble 30 of the carrier 27, BCB 28, and thesample 26 was placed on a 10° angled aluminum block (not shown in theDrawings). The whole ensemble 30 was heated to 100° C., in order toallow the sample 26 the ability to slide down on the slope of thealuminum block and self-align to the crystal axis of the carrier 27 whenthe BCB reached 100° C. where the BCB viscosity was reduced and thesample 26 became somewhat free to slidingly displace.

[0068] The ensemble 30 containing the carrier 27, the sample 26 and theBCB layer 28 holding them together aligned each to the other, was thenplaced on a quartz holder in a nitrogen-purged cube furnace that wasinitially at room temperature to be cured therein. Curing was performedin the following sequence:

[0069] A. Temperature ramp rising from 25C to 100C in 60 minutes, with asoak for 30 minutes;

[0070] B. Temperature ramp rising from 100C to 150C in 30 minutes, witha soak for 30 minutes;

[0071] C. Temperature ramp from 150C to 200C in 30 minutes, with a soakfor 2 hours;

[0072] D. Cool down the chamber manually to room temperature to avoidthermal shock when removing the ensemble 30 from the furnace.

[0073] After curing of the ensemble 30 as depicted in FIG. 6A, thesubstrate 11 was removed using either a chemo-mechanical(bromine-methanol) process or an acid etch process. The ensemble 30 wasmounted on a quartz disk with wax if the removal step used, (1) thebromine/methanol technique, or (2) with clear wax if the removaloperation used acid technique.

[0074] The removal of the substrate 11 was performed in three steps:

[0075] A. Fast etch was performed with the chemo-mechanical process(Bromine/Methanol with lightly loaded motion across a soft pad or Teflonsurface) until the substrate 11 was about 100 μm thick (below thisthickness range the multilayered structure might crack). A 3% bromine inmethanol solution and moderate speed on a MiniMet polisher resulted in25 μm/min etch rate. The uniformity of this process is not critical dueto the highly selective etch stop to be used in the next step.

[0076] Another fast etch can be purely chemical using aH₂SO₄:H₂O₂H₂O(1:8:1) mixture to uniformly remove the substrate 11 downto a remaining thickness of 20 μm. The etch rate starts at 25 μm/min forthe first 10 minutes but then slows down to 10 μm/min. This will alsoslightly affect the GaAs carrier 27, but the overall result is muchbetter because of the improved removal uniformity. The two processes arenon-selective so the total thickness must be monitored to stop beforethe etch-stop layer 12 is removed.

[0077] B. A selective etch of GaAs substrate 11 (over InGaP or InGaAsetch—stop layer 12) was performed. The best selectivity (500) was foundfor the H₃P0 ₄:H₂O₂H₂O (3:1:5) mixture. The etch rate was about 2 μm perminute for these technological parameters.

[0078] C. A selective etch of InGaP or InGaAs (over GaAs) was performedto remove the etch stop layer 12. The best selectivity was obtained forCH₂COOH:HCI (20:1) solution. 1 minute 30 seconds was enough to removethe 100 nm thick etch stop layer 12. It is to be noted that the clearwax may slightly dissolve and that the BCB 28 may peel off the carrier27, where not proteced by the multi-layered structure 10. When some waxor unprotected BCB contaminated the surface, the ensemble 30 was removedfrom the acid mixture and a solvent clean was performed. Thecontamination can be rinsed, but if not removed, it will act as aprotection layer preventing further etching of the etch-stop layer 12.

[0079] After substrate 11 removal, the sample was moderately heated on ahotplate to melt the wax and allow of the ensemble 30 detachment fromthe quartz disc. The back of the ensemble 30 was swabbed and rinsed withappropriate organic solvent to remove any residual wax. The resultingstructure is shown in FIG. 6B.

[0080] The photolithographic process was then performed aligned to thealignment marks 18 using a photomask defining the “top level” featuresof waveguide device.

[0081] As can be seen, the core layer 16 is easily accessible, as wellas the core layer 14, for forming independent optical structures thereinwhich is an advantage of the manufacturing process of the presentinvention. Similar to the defining of “bottom level” waveguide features,in the forming of “top level” waveguide features, a photoresist layer 31was deposited on the cleared surface 32 of the cladding layer 13, asshown in FIG. 7A. A respective photomask defining the “top level”waveguide features, aligned to the alignment marks 18, was used todefine the “top level” pattern.

[0082] After exposure of the photoresist 31, through the mask, andsubsequent etching, the photoresist 31 was left in the areas that wereintended to form “top level” optical features 33 of the core layer 14.The sample was then etched using ICP etching to a depth of 0.95-1.0microns using the photoresist 31 as a mask. This etching step removedthe cladding layer 13, core layer 14, and a portion of the couplinglayer 15 as shown in FIG. 7B to form highly confined waveguide features33 in the remaining unetched areas of the photoresist layer 31.

[0083] After etching, the photoresist 31 was removed using a three-stepprocess similar to that described in previous paragraphs with thereference to the “bottom level” waveguide patterns formation. The samplewas then cleaned using a solvent rinse (acetone, methanol andisopropanol). This process was then followed by the application of theadhesion promoter (such as Dow Chemical Company AP3000 Cycllotenadhesion promoter), which was spun at 2000 rpm for a duration of 30seconds. A drop of BCB (Dow Chemical Cyclotene 3022-57) was applied tothe formed sample and spun at 5000 rpm for the duration of 30 seconds tocompletely encapsulate the formed sample of the integral waveguidedevice of the present invention.

[0084] The encapsulated sample was then cured using the sequence of thetemperature and soak regimen similar to that one described in previousparagraphs. After curing, the BCB edge beads were removed using areactive ion etching process with the device area of the sample shadowmasked. The desired result can also be achieved by cleaving away theedge bead region of the formed sample.

[0085] The sample was then mounted on a quartz disc with white wax orclear wax, with the device side waxed down to the quartz disc inpreparation for cleaning of the carrier 27. The carrier 27 was thinnedto a remaining thickness of 110-135 microns using either of the fastetch or a selective etch similar to the technique described in previousparagraphs with regard to removal of the substrate 11. After thinning,the sample was demounted from the quartz disc by moderately heating thesame on the hot plate to melt the wax and allow detachment of the samplefrom the quartz disc. The completely encapsulated optical integrateddevice 34 is shown in FIG. 7C.

[0086] It is to be understood that the devices 34 manufactured by themethod of the present invention are suited for large scale siliconintegrated circuit fabrication, and therefore, a plurality of suchindividual optical waveguide devices 34 may be made on the samesemiconductor wafer. These wafers can be further cleaved into devicebars using standard procedures such as those used for semiconductorlaser and waveguide devices. The device bats are then mounted onto brasssubmounts using silver epoxy, and both facets of the device bars areantireflection coated using a well-known electron beam evaporatedalumina films technique to suppress facet reflections as known to thoseskilled in the art.

[0087] As described above, the manufacturing process of the presentinvention provides a way to implement vertical coupling by applying theadvantages of projection photolithography, substrate removal and lowtemperature polymer bonding to achieve vertically coupled, tightlyconfined single transverse mode couplers, interferometers and ringresonators of high quality.

[0088] The process makes possible the fabrication of such singletransverse mode, highly confined components using widely availablesemiconductor processing equipment, such as I-line steppers and reactiveion etching, high density plasma (ECR or ICP) etching or chemicallyassisted ion beam etching (CAIBE) tools. It further removes therequirement for direct write elecron beam lithography or deep-UVphotolithography, which have previously been used for laterally coupled,highly confined components which were necessary to use due to the verysmall dimension of the lithographically defined lateral coupling gap.Still further the fabrication technique avoids the limitations ofresolution and alignment presented by infraredbackside-aligned-contact-photolithography which has made singletransverse mode highly confined waveguide couplers difficult to realize.

[0089] Although the technique of the present invention could also beused with a wafer fusion bonding technique, a method using a polymerbonding technique is shown to achieve very high quality results withoutthe high temperatures typically used to fuse wafers. Additionally, thebonding polymer is used which serves as a low index cladding material.The use of a bonding and substrate removal process for verticallycoupled structures provides complete patterning access to bothwaveguiding layers without incurring the planarization difficultiesencountered with a conventional single side patterning and redepositionapproach. The invention described herein further extends the fabricationtechnique to allow the use of conventional projection lithography toolsuseful for low cost mass production.

[0090] Using the fabrication process of the present invention, differentoptical integrated waveguide structures have been fabricated,particularly vertically coupled microring resonators, such as singlemode microring optical channel filters with Q's greater than 3000 and anon-resonance channel distinction greater than 12 db, as well as 1-by-4multiplexer/demultiplexer crossbar array with second order microringfilters exhibiting channel-to-channel crosstalk lower than 10 db.

[0091] Similar to the process described in previous paragraphs, withregard to FIGS. 3-7C, the fabrication began with the molecular beamepitaxy epilayer growth. The two 0.5 μm thick GaAs guiding layers wereseparated with a 0.5 μm Al_(0.5)Ga_(0.5)As coupling layer. Both guidinglayers had a 0.3 μm Al_(0.5)Ga_(0.5)As top cladding. The In_(0.48)Ga₅₂Petch-stop layer had 0.1 μm thickness for substrate removal by selectivewet etching. In the following steps, a 10×I-line stepper was used forphotolithography and the GaAs/AlGaAs features were etched in aninductive coupled plasma (ICP) system for smooth and vertical sidewalls.First, the alignment keys were etched for a depth of 2.1 μm in order toreach the InGaP layer. These keys were then used for processing bothsides of the epilayer structure.

[0092] Subsequently, the waveguide layer was exposed and etched for atotal of 0.95 μm. An organic polymer BCB was used to bond the sample toa GaAs transfer substrate (carrier). The BCB thickness was typicallybetween 1 and 2 μm. The sample substrate was then removed by selectivewet etching to the stop etch layer, which was afterwards removed,exposing the second GaAs/AlGaAs layer for processing. The“etched-through” alignment keys were further used to align the microringlayer, which was etched for a depth of 0.95 μm. The sample was thenencapsulated with BCB to insure refractive index profile symmetry.Finally, the sample was thinned and cleaved for high quality facets andan Al₂O₃ antireflection coating was deposited on input and outputfacets.

[0093] The resulting structure for a single microring optical channeldropping filter is shown in FIG. 8. The filter has a microring 35(vertically coupled to waveguides positioned on the other level of thestructure and thus not seen in the Drawings), input port 36, throughputport 37, and drop port 38. The use of BCB provides a low refractiveindex (1.53-1.55) layer between the GaAs transfer substrate (carrier)and the epilayer that makes tightly confined waveguides possible on bothdevice layers. Furthermore, the BCB encapsulation provides a symmetricrefractive index structure required to have identical microringstructures on both layers for multi-ring resonators for example, asshown in FIG. 9. This device has multiple of microrings 39, input port40, throughport 41, and drop ports 41-44. The waveguides 45 arepositioned on the opposite level of the device, and thus, are shown inhidden lines in FIG. 9. A thin (approximately 0.2 μm) AlGaAs couplinglayer is left to reduce the effect of layer-to-layer misalignment.

[0094]FIG. 10 shows the normalized coupling coefficient as a function ofthe layer-to-layer misalignment based on the mode overlap integral. Thedesign study shows that when a 0.3 μm coupling layer is left, thecoupling coefficient drops by 10% for a 0.15 μm misalignment, as opposedto 10% coupling coefficient drop for 0.08 μm misalignment when there isno coupling layer left. However, to limit leakage loss in microrings,0.3 μm is generally the maximum tolerable thickness one can leave fornegligible bending loss. This design also provides added mechanicalstrength to the structure for the BCB encapsulation. It has beenobserved experimentally that those areas without the coupling layerundergo stress when the encapsulating BCB is cured at 200° C., so thatbroken waveguides and microrings may result. Finally, the alignmentscheme using a stepper and “etched-through” alignment keys provides fora high quality layer-to-layer alignment with an error kept below 0.25μm.

[0095] Device responses were obtained by coupling light from an externalcavity laser diode into the input waveguides and collecting the variousoutputs with optical fibers. The fabricated devices of FIGS. 8 and 9have waveguide widths of 0.7 μm and a mid-layer thickness of 0.2 μm. Thewaveguides are tapered in and out to 2.5 μm width to improve fibercoupling efficiency.

[0096] Fabricated single microring resonators of FIG. 8 have radii of2.5, 5 and 10 μm. Resonant dropped power was observed for all radii,however the 2.5 μm microrings have low Q's due to high round trip lossin the resonators. A wide scan of the 10 μm microring TM spectrum isshown in FIG. 11 that has a Q greater than 3000 and TM and TE bandwidthsof 0.5 and 0.59 nm respectively (FIG. 12A). The on-resonance extinctionand the dropping efficiency were similar for both TE and TMpolarizations. The free spectral range is 11.14 nm for TM and 11 nm forTE. The resonance peak shapes are shown in FIG. 12A, shifted inwavelength. The on-resonance extinction on the throughput port 37 ofFIG. 8 is greater than 12 dB (FIG. 12B), and propagation losses in thering 35 are lower than 17 cm⁻¹ for the TM mode. Finally, the droppingefficiency is about 60% for both TE and TM but this measurement assumesreproducible coupling efficiency at the drop port 38 and throughput port37. All-pass racetrack resonators were also made and exhibited Q'sgreater than 6000.

[0097] A 1-by-4 MUX/DEMUX was fabricated with a crossbar arrayarchitecture to optimize device integration. Typically, a tightlyconfined waveguide crossing suffers from scattering and crosstalk. Sincein the architecture of the present invention the waveguides are ondifferent layers, no additional loss was measured experimentally. Thecrosstalk was smaller than −38 dB. Moreover, double microrings are usedas filter blocks for dropping wavelengths with lower channel-to-channelcrosstalk due to the sharper roll-off of second-order filters.

[0098] The results are shown in FIG. 13 measured for the four dropchannels 41-44 of FIG. 9. The ring radius was 5 μm with free spectralrange of 22.5 nm with a TM bandwidth of 0.9 nm. However, because offabrication imprecision, the waveguides in the two layers had slightlydifferent widths which resulted in a 3 nm detuning between the upper andlower rings. The resulting second-order resonance is therefore broad anddouble-peaked. Nevertheless, a second-order roll-off was observed andchannel-to-channel crosstalk lower than −10 dB was measured betweenadjacent channels.

[0099] In summary, a new fabrication process has been developed which isused to create working prototypes of vertically coupled microringresonator devices. One of the key improvements over the prior art is theability to create device patterns using a conventional I-line projectionstepper. This allows the production of single transverse mode highlyconfined waveguides on semiconductors, where the required lateraldimensions are in the submicron range, using a lithography system forlarge-scale silicon integrated circuit fabrication. The alignment schemeusing a stepper and “etched-through” alignment keys provides a highquality layer-to-layer alignment with an error kept below 0.25 μm.

[0100] The alignment tolerance issue was addressed in the presentinvention via a design innovation, whereby the alignment tolerance isgreatly increased due to the presence of the coupling layer which wasnot completely etched through.

[0101] The procedure and structure described above achieves additionaladvantages for high quality device fabrication. The use of a fullysymmetric structure with the initial substrate and carrier consisting ofidentical (or very similar) materials, and symmetric polymerencapsulation contributes to a high quality bonded structure byminimizing stress due to material mismatch.

[0102] The symmetric polymer encapsulation also improves the matching ofproperties of the resonators and waveguides on the two waveguidinglayers. Specifically, the new fabrication process has been demonstratedfor vertically coupled semiconductor microring resonators in which themid-layer and the “etched-through” alignment keys helped prevent filterresponse degradation from layer-to-layer misalignment. Moreover, therefractive index profile symmetry of the bonded and encapsulatedepilayer structure permits the use of tight confinement on bothwaveguiding layers which makes possible the fabrication of identicalresonators for high-order filtering. High-Q microrings and a 1-by-4MUX/DEMUX crossbar array exhibiting a sufficient second-orderdemultiplexing response, were fabricated using the technique of thepresent invention.

[0103] The low index polymer undercladding provides the additionalbenefit of reducing leakage loss of light into the carrier material.However, the concepts for alignment marks and alignment tolerant designdescribed herein may also be implemented in non-polymersubstrate-removal device fabrication processes which use direct waferbonding or fusing techniques.

[0104] A number of advantages of the vertical coupling design approachthat apply to the implementation of the present invention are asfollows:

[0105] 1. The higher degree of vertical confinement and isolation fromthe substrate relative to prior lateral coupling design allows areduction in the etching depth required for low optical radiation loss.The reduction of etching depth reduces the amount of etch mask erosionduring the waveguide-defining etch process, potentially reducing theetched sidewall roughness and optical scattering loss.

[0106] 2. The design geometry is favorable to creating active deviceswith electrodes to contact doped regions used in electro-optic orgainproducing structures. The remaining coupling layer described abovelends the additional advantage of providing a surface to use forelectrically contacting the coupling layer, which could be doped.Contacts could be created in the open regions adjacent to unetched ridgestructures. 3. The structure allows each of the two waveguiding cores tobe individually optimized in dimension and composition for itsparticular function. For example, one core may be active with the otherbeing passive.

[0107] As described above, the invention described herein permits aneasier and higher quality implementation of vertically coupled devicesin highly confined optical waveguides. Although this invention has beenimplemented and demonstrated to work in a single material system, itequally may be implemented in a number of optical waveguiding materialssystems where high quality tightly confined waveguides can be made andwhere suitable bonding agents and selective etching recipes exist. Thecombination of the new alignment mark creation method for substrateremoval processes and the increased alignment tolerance of the devicedesign with a remaining mid-layer makes it possible to fabricatevertically coupled, highly confined, single transverse mode opticalwaveguide devices with conventional projection lithography in a processcompatible with low cost mass production.

[0108] Although this invention has been described in connection withspecific forms and embodiments thereof, it will be appreciated thatvarious modifications other than those discussed above may be resortedto without departing from the spirit or scope of the invention. Forexample, equivalent elements may be substituted for those specificallyshown and described, certain features may be used independently of otherfeatures, and in certain cases, particular locations of elements may bereversed or interposed, all without departing from the spirit or scopeof the invention as defined in the appended claims.

What is claimed is:
 1. A method for fabrication of vertically coupledintegrated photonics devices, including the steps of: (a) forming amulti-layered structure having first and second levels and comprising:at least a first and a second waveguiding core layers, a coupling layerdisposed between said at least first and second waveguiding core layers,and first and second cladding layers sandwiching said waveguiding corelayers therebetween; (b) forming alignment marks extending substantiallythrough said multi-layered structure at predetermined areas thereof, (c)forming first features at said first level of said multi-layeredstructure, said first features extending through said first cladding,said first waveguiding core layer and partially through said couplinglayer; and (d) forming second features at said second level of saidmulti-layered structure, said second features extending through saidsecond cladding, said second waveguiding core layer and partiallythrough said coupling layer; whereby said first and second features aredisposed at predetermined location with reference to said alignmentmarks identifiable at both said first and second levels of saidmulti-layered structure.
 2. The method of claim 1, wherein in said steps(c) and (d), said first and second features are defined by means of aprojection lithography technique.
 3. The method of claim 2, wherein ineither of said steps (c) and (d), said projection lithography techniquecomprising the steps of: depositing a photoresist on the externalsurface of a respective one of said cladding layers, exposing saidphotoresist through a projection stepper prealigned with respect to saidalignment marks, and patterning a respective one of said waveguidingcore layers.
 4. The method of claim 3, further comprising the steps of:patterning said respective waveguiding core layer by means of etchingthrough said respective cladding layer, said respective waveguiding corelayer, and through portion of said coupling layer.
 5. The method ofclaim 1, further comprising the steps of: in said steps (c) and (d),penetrating into said coupling layer to a predetermined depth to leave aportion of said coupling layer separating first and second featuresdefined in said first and second waveguiding core layers, respectively.6. The method of claim 5, wherein said portion of said coupling layer isthinner than 0.3 μm.
 7. The method of claim 1, further comprising thesteps of: forming said multi-layered structure on a surface of asubstrate structure, said substrate structure comprising a substratelayer and a stop-etch layer positioned between said substrate layer anda respective one of said cladding layers.
 8. The method of claim 7,further comprising the steps of: subsequent to the completion of saidstep (c), adhering said multi-layered structure to a carrier structurein a predetermined alignment therewith with said first featuresextending towards said carrier structure.
 9. The method of claim 8,wherein in the step of adhering, said multilayered structure is adheredto said carrier by a low temperature polymer bonding.
 10. The method ofclaim 9, wherein in said low temperature polymer bonding, said polymeris benzocyclobutene polymer.
 11. The method of claim 8, furthercomprising the steps of: (E) upon completion of said step (d), removingsaid substrate structure.
 12. The method of claim 11, further comprisingthe steps of: performing said step (e) after completion of said step(b); encapsulating said second level of said multi-layered structureinto a polymer, and removing said carrier.
 13. The method of claim 7,further comprising the steps of: after said step (b), forming metalfilms on said stop-etch layer within said alignment marks.
 14. Avertically coupled integrated photonics device, comprising: amulti-layered structure including: at least a pair of waveguiding corelayers independently patterned to define respectively first and secondwaveguiding features therein, a coupling layer spacing said first andsecond waveguiding features, a pair of cladding regions sandwiching saidfirst and second waveguiding features therebetween, and a polymerenvelope encapsulating said multilayered structure therewithin.
 15. Thevertically coupled integrated photonics device of claim 14, wherein thethickness of said coupling layer is less than approximately 0.3 μm. 16.The vertically coupled integrated photonics device of claim 14, whereinsaid first waveguiding features include at least one microring.
 17. Thevertically coupled integrated photonics device of claim 14, wherein saiddevice includes a single microring resonator add/drop filter.
 18. Thevertically coupled integrated photonics device of claim 14, wherein saiddevice includes a double microring resonator.
 19. The vertically coupledintegrated photonics device of claim 14, wherein said device includesGaAs/AlGaAs materials systems.
 20. The vertically coupled integratedphotonics device of claim 14, wherein said polymer envelope has a lowrefractive index in the range of 1.53-1.55.